John Lewis has released an early hint of its Christmas ad with a new campaign centred on its recently-revived 'Never Knowingly Undersold' price pledge.The 'through the decades' ad is the first in a three-part campaign for the all-important retail 'Golden Quarter', which will culminate with the department store's highly-anticipated Christmas spot.The advert features archive footage and focuses on a single store window changing over a century as it is dressed and redressed with products such as fashions of the roaring 1920s and a toaster so innovative that that it took centre stage in 1925.